Computer systems need a way to store and retrieve data from a variety of data devices, such as disk drives, printers, display screens, and scanners. In the past, each computer typically had its own directly-attached devices, which no other computer was capable of using. But, this was a cumbersome and expensive design because sharing data among computers was difficult and a particular device might stay idle and unused for lengthy periods.
In order to overcome the aforementioned problems, computers are connected in networks, and one computer (often called a client or a host) is allowed to store and retrieve data from another computer's (often called a server) devices. Since many clients or hosts (initiators) may wish to access the same device, the device needs a way to distinguish the initiators from each other so that, for example, if one initiator is issuing a write command to a tape device, another initiator is not allowed to issue a rewind command to the same tape device. Prior devices have attached to multiple physical ports at a server with different initiators allocated to different physical ports. But, multiple physical ports require duplicate hardware, which is expensive. What is needed is a less expensive solution that allows a device to distinguish between multiple initiators.